Well, as far as character design goes, I'd have to say Akuma, just for sheer coolness. Gameplay wise, I'd have to say either Guy (Final Fight! W00t!) or Rolento (Again, Final Fight!). Guy's got some insane speed and sneaky moves. Rolento, well, he's extremely versatile.

(Can't wait for Sammy vs Capcom. Akuma vs Sol Badguy? Hell yeah!)